The invention concerns a method for contactfree drying of a paper or board web, in which method both infrared radiation and drying air jets are used for drying, by means of which these air jets the web passing through the dryer is, at the same time, supported free of contact, preferably from two sides, in which method the web is passed first into an infrared drying gap or gaps and thereupon into an airborne web-drying gap, and in which method the infrared radiators in the infrared drying unit or units are cooled by means of air currents.
The present invention relates to the drying of a paper web, board web, or of any other, corresponding moving web. A typical application of the invention is the drying of the paper web in connection with its coating or surface-sizing.
As is known in prior art, paper webs are coated either by means of separate coating devices or by means of on-machine devices integrated in paper machines or by means of surface-sizing devices, which operate in the drying section of a paper machine so that the web to be coated is passed, at the final end of a multi-cylinder dryer, to a coating device, which is followed by an intermediate dryer and finally, e.g., by one group of drying cylinders as an after-dryer. A typical application of the present invention is the said intermediate dryer placed after a coating device, to which the invention is, however, not confined.
In prior art, so-called airborne web dryers are known, wherein a paper web, board web or equivalent is dried free of contact. Airborne web dryers are used, e.g., in paper coating devices after a blade, roll or spread coater to support and to dry the web that is wet with the coating agent, free of contact. In airborne web dryers, different blow nozzles for the supporting and drying air as well as arrangements of same are applied. Said blow nozzles can be divided into two groups, i.e. positive-pressure or float nozzles and negative-pressure or foil nozzles, both of which can be applied in the method of the present invention.
The prior-art airborne web dryers that are used most commonly are based exclusively on air blows. It is partly for this reason that the airborne web dryer becomes quite spacious, because the distance of effect of the airborne web dryer must be relatively long in order that a sufficiently high drying capacity could be obtained. Another reason for these drawbacks is that in air drying the depth of penetration of the drying remains relatively low.
In the prior art, different dryers are known which are based on the effect of radiation, in particular of infrared radiation. The use of infrared radiation provides the advantage that the radiation has a relatively high depth of penetration, which depth of penetration is increased when the wavelength becomes shorter. The use of infrared dryers in the drying of paper webs has been hampered, e.g., by the risk of fire, because the temperatures in infrared radiators become quite high, e.g. 2000.degree. C., in order that a drying radiation with a sufficiently short wavelength could be achieved.
Electric infrared dryers as separate or as exclusively used are also energy-economically unfavourable owing to the relatively high cost of electric energy, as compared, e.g., with natural gas.
In paper coating stations, including on-machine coating stations, separate infrared dryers have been used whose drying is based exclusively on the radiation effect. However, by means of these infrared dryers, a sufficiently good adjustability of paper quality and evaporation has not been obtained. Moreover, the drying process becomes highly dependent on the operational quality of the infrared dryer.